An olefin (e.g., propylene) has been polymerized using an olefin polymerization catalyst. In particular, a propylene-based block copolymer that is obtained by effecting homopolymerization of propylene (or random copolymerization of propylene and a small amount of ethylene) in the first step, and effecting copolymerization of propylene and ethylene (or propylene and another α-olefin) in the second step, may be melted, molded using a molding machine, a stretching machine, or the like, and used for a variety of applications (e.g., automotive part, home appliance part, container, and film). In particular, since a propylene-ethylene block copolymer exhibits excellent mechanical properties (e.g., rigidity and heat resistance), and can be produced relatively inexpensively, a propylene-ethylene block copolymer has been used for a wide range of applications.
A solid catalyst component that includes magnesium, titanium, an electron donor compound, and a halogen atom as essential components has been known as a component of the olefin polymerization catalyst. A number of olefin polymerization catalysts that include the solid catalyst component, an organo aluminum compound, and an organosilicon compound, have been proposed.
A propylene-based block copolymer has been used by preference since the balance between rigidity and impact resistance is good. An olefin polymerization catalyst is required to have a capability to produce polypropylene that exhibits high stereoregularity in order to achieve a good balance between rigidity and impact resistance. In order to achieve high impact resistance, an olefin polymerization catalyst is required to have a capability to produce at least a specific amount of copolymer while ensuring high second-step copolymerization activity, and produce a copolymer with high randomness while ensuring high sustainability of polymerization activity and excellent controllability of the polymerization reaction.
A propylene-based block copolymer is a blend of a polymer component that mainly includes propylene, and a random copolymer component obtained by subjecting propylene and an α-olefin monomer (e.g., ethylene) to random copolymerization, and is normally produced by multistep polymerization that sequentially effects polymerization under conditions corresponding to each component to blend each component in the reactor. The propylene-based block copolymer is typically used for injection molding applications (e.g., automotive bumper). In recent years, it has been desired to increase the melt flow rate (MFR) of the propylene-based block copolymer in order to improve the productivity of the injection molding process. The MFR of the propylene-based block copolymer is uniquely determined by the MFR of the polymer component that includes propylene, the MFR of the random copolymer component obtained by subjecting propylene and an α-olefin monomer (e.g., ethylene) to random copolymerization, and the content of the random copolymer component in the block copolymer. It is necessary to increase the MFR and the content of the random copolymer component to a level equal to or higher than a given level in order to improve the quality (particularly impact strength) of the propylene-based block copolymer. It is necessary to effectively introduce the α-olefin (particularly ethylene) into the random copolymer part, and relatively reduce the content of crystalline polyethylene in order to maintain high quality. Therefore, development of a technique that achieves relatively high polymerization activity when producing the random copolymer part (rubber part) obtained by subjecting propylene and the α-olefin to random copolymerization, and efficiently introduces the α-olefin (e.g., ethylene) into the random copolymer part, has been desired.
The propylene-based block copolymer is required to exhibit impact strength when used for injection molding applications. In particular, the propylene-based block copolymer is required to exhibit improved low-temperature impact strength when used to produce an automotive bumper or the like. The low-temperature impact strength of the propylene-based block copolymer depends on the brittle temperature of the random copolymer component. If the propylene content in the random copolymer component is too high, the brittle temperature of the random copolymer component increases, and the low-temperature impact strength of the propylene-based block copolymer becomes insufficient. It is necessary to decrease the brittle temperature of the random copolymer component in order to increase the low-temperature impact strength of the propylene-based block copolymer. It has been considered that it is desirable to increase the α-olefin (e.g., ethylene) content in the random copolymer component in order to decrease the brittle temperature of the random copolymer component.
The propylene-based block copolymer production process has been improved from the viewpoint of implementing a simplified process, a reduction in production cost, an improvement in productivity, and the like. When the propylene-based block copolymer was initially produced on an industrial scale, it was necessary to remove a catalyst residue and an atactic polymer from the resulting propylene-based block copolymer since the performance of the catalyst was low, and a slurry polymerization process that utilizes a solvent or the like was mainly used. At present, a gas-phase polymerization process is mainly used along with a remarkable improvement in the performance of the catalyst. In particular, a gas-phase polymerization process that removes the heat of polymerization by utilizing the latent heat of liquefied propylene is advantageous in that high heat removal performance can be achieved using small-scale equipment.
The propylene-based block copolymer is normally produced by producing a polymer component (a) that mainly includes propylene in the first polymerization step, and producing a random copolymer component (b) in the second polymerization step by subjecting propylene and an α-olefin (e.g., ethylene) to random copolymerization. If the residence time distribution of the polymer particles that have been obtained by the first polymerization step and are subjected to the second polymerization step is wide, the reactor used for the second polymerization step may be fouled, or the impact strength of the block copolymer (product) may decrease. It is considered that such a problem occurs since the activity of the polymer particles that are subjected to the second polymerization step varies to a large extent due to the wide residence time distribution, and the amount of particles that produce the random copolymer component in the second polymerization step increases to a large extent. Therefore, it is desired to develop a production method that ensures that the polymer particles that are subjected to the second polymerization step exhibit high activity when polymerizing the random copolymer component, and the residence time is short (i.e., the residence time distribution is narrow). Accordingly, it is desired that the catalyst used for producing a propylene-based block copolymer exhibit relatively high activity during random copolymerization.
When producing polypropylene, hydrogen that has a capability to cause a chain transfer reaction is normally used as a molecular weight modifier. It is necessary to use hydrogen at a higher concentration in order to produce polypropylene having a higher MFR (i.e., lower molecular weight). When producing polypropylene having a high MFR using the gas-phase polymerization process that utilizes the latent heat of liquefied propylene, there is a tendency that the hydrogen concentration in unreacted gas increases, and the dew point decreases since hydrogen is used at a high concentration. As a result, productivity decreases due to removal of heat. A similar problem occurs when producing a random copolymer component having a high comonomer content using a comonomer having a low dew point (e.g., ethylene). Specifically, there is a tendency that the comonomer concentration in unreacted gas increases, and the dew point decreases since the comonomer is used at a high concentration. In this case, the heat removal performance in the recycle system becomes insufficient. When producing a propylene-based block copolymer having a high MFR, the heat removal performance and productivity decrease to a large extent in the first polymerization step. When producing a propylene-based block copolymer having a high ethylene content, the heat removal performance and productivity decrease to a large extent in the second polymerization step. In order to solve this problem, it is desirable that polypropylene having a high MFR can be produced at a lower hydrogen concentration, and a random copolymer component having a high ethylene content can be produced at a lower ethylene concentration. When the hydrogen concentration or the ethylene concentration is low, the hydrogen concentration or the ethylene concentration in unreacted gas decreases, and a decrease in dew point can be suppressed, so that productivity can be improved.
Several methods have been proposed that solve the above problem by improving the polymerization catalyst. For example, a method that utilizes a catalyst for which the hydrogen response is improved by utilizing an aluminum halide when producing the solid catalyst (see Patent Document 1), a method that utilizes an organoaluminum component and an organozinc component as a promoter (see Patent Document 2, for example), a method that utilizes an organosilicon compound that includes an amino group (see Patent Documents 3 to 5, for example), and the like have been proposed as a method that solves the problem of producing polypropylene having a high MFR. The olefin polymerization catalyst disclosed in Patent Document 1 exhibits excellent activity with respect to hydrogen (hydrogen response) as compared with known polymerization catalysts, and an olefin polymer obtained using the solid catalyst component disclosed in Patent Document 1 exhibits high fluidity (MFR) when melted, and is particularly useful when producing a large molded article by injection molding or the like. However, a catalyst that exhibits high activity when producing the random copolymer part, and can efficiently incorporate ethylene in the random copolymer part, has not yet been obtained. A method that utilizes a titanium compound that includes a Ti—N linkage (see Patent Document 6, for example), a method that utilizes an organosilicon compound and a saturated hydrocarbon during second-step polymerization (see Patent Document 7, for example), and the like have been proposed as a method that solves the problem of improving the copolymerizability of ethylene. However, an improvement in copolymerizability is still insufficient.